1. Field of the Invention
The present invention is broadly concerned with chemically modified starches highly resistant to .alpha.-amylase digestion, food products containing such modified starches, and methods of forming the starches. More particularly, the invention pertains to such starches wherein the starches are of the RS.sub.4 variety and exhibit at least about 20% resistance to .alpha.-amylase digestion; these starches may be incorporated into yeast- or chemically-leavened food products, such as breads and crackers to provide a low calorie source of dietary fiber. The starches hereof are preferably prepared by cross-linking thereof using a multifunctional phosphorylating agent (e.g., sodium trimetaphosphate (STMP) or a mixture of STMP and sodium tripolyphosphate (STPP)).
2. Description of the Prior Art
Starch serves as a food reserve in plants, and it is an important component in the human diet. The digestion of starch is mediated by salivary and pancreatic .alpha.-amylase, which catalyze the formation of maltose, maltotriose and dextrins. The latter products are further hydrolyzed to D-glucose in the brush border of the small intestines. .alpha.-Amylases (MW 50,000-60,000 Daltons) are endo-acting enzymes that catalyze the hydrolysis of the .alpha.-1,4 bonds in the amylose and amylopectin molecules that comprise starch; they do not hydrolyze the .alpha.-1,6-bonds but can by-pass them. Glucoamylase and .alpha.-glucosidase are exo-acting enzymes that cleave both .alpha.-1,4 and .alpha.-1,6 linkages between D-glucose.
In the early 1980's it became apparent that some starch resists digestion. Instead, it enters the colon where it is fermented by bacteria. The resistance of starch to digestion in the upper GI tract is recognized to depend on intrinsic factors, which include the physical state of a food and its preparation and storage, and on extrinsic factors, which are the physiological conditions influencing starch digestion. Starch entering the colon exerts a number of different physiological effects (see below) compared to just one in the upper gastrointestinal tract, namely production of D-glucose to provide energy.
In 1987 Englyst and Cummings at the MRC Dunn Clinical Nutrition Center in Cambridge, UK, proposed a classification of starch based on its likely digestive properties in vivo. They also devised in vitro assay methods to mimic the various digestive properties of starch. Three classes of dietary starch were proposed:
(1) Rapidly Digestible Starch (RDS). RDS is likely to be rapidly digested in the human small intestine; examples include freshly cooked rice and potato, and some instant breakfast cereals.
(2) Slowly Digestible Starch (SDS). SDS is likely to be slowly yet completely digested in the small intestine; examples include raw cereal starch and cooked pasta.
(3) Resistant Starch (RS). RS is likely to resist digestion in the small intestine. RS is thus defined as the sum of starch and starch degradation products not likely to be absorbed in the small intestine of healthy individuals. RS can be subdivided into four categories depending on the causes of resistance (Englyst et al 1992; Eerlingen et al 1993).
RS.sub.1. Physically inaccessible starch due to entrapment of granules within a protein matrix or within a plant cell wall, such as in partially milled grain or legumes after cooing. PA1 RS.sub.2. Raw starch granules, such as those from potato or green banana, that resist digestion by .alpha.-amylase, possibly because those granules lack micropores through their surface. PA1 RS.sub.3. Retrograded amylose formed by heat/moisture treatment of starch or starch foods, such as occurs in cooked/cooled potato and corn flake. PA1 RS.sub.4. Chemically modified starches, such as acetylated, hydroxypropylated, or cross-linked starches that resist digestion by alpha-amylase. Those modified starches would be detected by the in vitro assay of RS. However, some RS.sub.4 may not be fermented in the colon.
RS.sub.1, RS.sub.2, RS.sub.3 are physically modified forms of starch and become accessible to .alpha.-amylase digestion upon solubilization in sodium hydroxide or dimethyl sulfoxide. RS.sub.4 is chemically modified and remains resistant to .alpha.-amylase digestion even if dissolved.
RS is of increasing interest as a food ingredient. Unlike common dietary fiber sources, RS does not hold much water and, thus may be a preferred fiber source for use in low moisture products such as cookies and crackers. Also, RS is free of a gritty mouthfeel, and unlike traditional fiber sources does not significantly alter flavor and textural properties of foods. Those characteristics can improve the processing and quality of foods such as baked and extruded products when RS is added. Furthermore, RS constitutes dietary fiber, and may be assigned zero calories.
RS is counted with the dietary fiber fraction of food and is believed to function as fiber in the human digestive tract. The reduced bioavailability of RS in the human gastrointestinal tract has significant physiological effects, such as slow glucose release and a lower postprandial glycemic response with lower blood lipids. When RS reaches the colon it is fermented to hydrogen, methane, carbon dioxide, lactic acid (transient), and short chain fatty acids (acetate, propionate, and butyrate)with purported beneficial effects that suggest prevention of colonic diseases.
It is known that the digestibility of starches can be affected by processing and storage conditions. Chemical modification of starches has been shown to inhibit their in vitro digestibility, with the extent of inhibition related to the degree of modification and presumably the type of modification. The variation depends on the botanical origin of the starch, the modifying agent(s) used and the subsequent chemical bonds and derivatives formed, the extent of granule gelatinization, and choice of enzyme (Anonymous 1972, Filer 1971). Banks et al (1973) demonstrated that the degree of substitution determined the rate and extent of amylolytic attack on hydroxyethyl amylose.
Leegwater and Luten (1971) reported an exponential decrease in the digestibility of hydroxypropyl substituted starches by pancreatin with an increasing degree of substitution up to 0.45% HP. Janzen (1969) reported that potato starch phosphate cross-linked with 0.05 and 0.1% POCl.sub.3 has no influence on the in vitro digestion with pancreatin as determined by the weight of residue after digestion. Modification with 0.5 and 1.5% POCl.sub.3, however, inhibits the hydrolysis considerably. Hood and Arneson (1976) have reported that hydroxypropyl distarchphosphate modification increases the digestion of ungelatinized starch but decreases the digestion of gelatinized starch. Introduction of cross-links tends to stabilize granule structure and restrict the degree of swelling. With a high degree of cross-linking the porosity of the gel phase of a granule will be too fine to admit large molecules. Some reports have indicated that phosphate cross-linking slightly reduces enzymatic hydrolysis or has no effect on hydrolysis when compared to the unmodified starch (Anonymous 1972, Ostergard, 1988; Bjorck et al., 1988). Changes in the intestinal microflora of rats eating hydroxypropyl distarch phosphate, hydroxypropyl starch, and distarch phosphate suggest that starches containing ether linkages are more difficult to digest than those containing only phosphate linkages (Hood, 1976). A hydroxypropyl distarch phosphate derivative of potato starch exhibits 50% in vivo digestibility in rat (Bjorck et al., 1988).
In view of the known health benefits of dietary fibers in general, and the potentially advantageous additional properties of RS.sub.4 starches in food products, there is a need in the art for improved RS.sub.4 starches having a high degree of resistance to .alpha.-amylase digestion, as well as low-cost methods of producing such chemically modified starches.